Overexpression of rainbow trout estrogen receptor domains in Escherichia coli: characterization and utilization in the production of antibodies for immunoblotting and immunocytochemistry

Direct and Indirect Effects of Sex Steroids on Gonadotrope Cell Plasticity in the Teleost Fish Pituitary

The pituitary gland controls many important physiological processes in vertebrates, including growth, homeostasis, and reproduction. As in mammals, the teleost pituitary exhibits a high degree of plasticity. This plasticity permits changes in hormone production and secretion necessary to meet the fluctuating demands over the life of an animal. Pituitary plasticity is achieved at both cellular and population levels. At the cellular level, hormone synthesis and release can be regulated via changes in cell composition to modulate both sensitivity and response to different signals. At the cell population level, the number of cells producing a given hormone can change due to proliferation, differentiation of progenitor cells, or transdifferentiation of specific cell types. Gonadotropes, which play an important role in the control of reproduction, have been intensively investigated during the last decades and found to display plasticity. To ensure appropriate endocrine function, gonadotropes rely on external and internal signals integrated at the brain level or by the gonadotropes themselves. One important group of internal signals is the sex steroids, produced mainly by the gonadal steroidogenic cells. Sex steroids have been shown to exert complex effects on the teleost pituitary, with differential effects depending on the species investigated, physiological status or sex of the animal, and dose or method of administration. This review summarizes current knowledge of the effects of sex steroids (androgens and estrogens) on gonadotrope cell plasticity in teleost anterior pituitary, discriminating direct from indirect effects.

A 45-years journey within the reproductive brain of fish

This article summarizes the scientific carrier of Dr. Olivier Kah, currently emeritus research director at the National Center of Scientific Research (CNRS) in France. Olivier Kah partly grew up in Africa where he developed a strong interest for animals. He studied biology in Paris and Bordeaux. He next received his PhD at the University of Bordeaux en 1978 and his Doctor of Science degree in 1983. He joined the CNRS in 1979 until his retirement in 2016. Olivier Kah dedicated his carrier to the study of reproduction, in particular to the roles of brain neuropeptides and neurotransmitters in the control of the reproductive axis in vertebrates, mostly fish. More specifically, Olivier Kah was specialized in the use of morphofunctional techniques that he implemented to the study of the organization of the hypothalamo-pituitary complex. He was also interested in the steroid feedback and studied intensively the expression and regulation of estrogen and glucocorticoid receptors in the rainbow trout and the zebrafish. In the last 10 years, Olivier Kah's team focused on the expression and regulation of aromatase in the brain and established that aromatase expression is restricted to a unique brain cell type, the radial glial cells, which serve as progenitors during the entire life of fish. He is also interested in the impact of endocrine disruptors using the zebrafish as a model and recently his team has developed an exquisitely sensitive in vivo assay to screen estrogenic chemicals on zebrafish embryos.

New insights into the factors mediating the onset of puberty in sea bass

In populations of 1-year-old male European sea bass (Dicentrarchus labrax), only large males are able to acquire for the first time a functional competence of their reproductive axis; in other words, to attain puberty. To examine the causes and mechanisms involved in the onset of puberty in this species, a size sorting sampling was carried out to obtain two experimental groups of small and large male fish exhibiting different growth rates. As expected, only large fish reached full spermiogenesis (stage V of testicular development) by the end of the experiment. Our study suggests that fish size is a permissive condition to ensure full effectiveness of the hormonal (Gnrh1, gonadotropins and sexual steroids) actions. Thus, though small fish had endocrine profiles similar to those of large fish, their amplitude was much lower, and was most likely the reason why functional competence of the reproductive axis was not achieved. Moreover, this work provides evidence of the involvement of kisspeptin and Gnrh1 systems in the onset of puberty in a marine teleost fish. It also indicates that very likely kisspeptin and Gnrh1 may regulate gonadotropins and sex steroids at specific stages of testicular development.

Investigation of the functional properties and subcellular localization of alpha human and rainbow trout estrogen receptors within a unique yeast cellular context

Estrogens are steroid hormones that play a pivotal role in growth, differentiation and function of reproductive and non-reproductive tissues, mediated through estrogen receptors (ERs). Estrogens are involved in different genomic and non-genomic cell signaling pathways which involve well-defined subcellular ER localizations. Thus, ER activity results from complex interplays between intrinsic binding properties and specific subcellular localization. Since these two factors are deeply intricate, we carried out, in a unique yeast cell context, a comparative study to better understand structure/function/subcellular distribution relationships. This was carried out by comparing two ERs: the human ER α subtype (hERα) and the short form of the α isoform of the rainbow trout ER (rtERαS). Their distinct binding properties to agonist and antagonist ligands and subcellular localizations were characterized in Saccharomyces cerevisiae yeast cells. An unexpected partial agonistic effect of ICI 182-780 was observed for rtERαS. Concomitant to distinct binding properties, distinct subcellular localizations were observed before and after ligand stimulation. Due to the unique cell context, the link between ERs intrinsic binding properties and subcellular localizations is partly unveiled and issues are hypothesized based on the role of cytoplasmic transient complexes which play a role in the ER cytoplasmic/nuclear partition, which in turn is critical for the recruitment of co-regulators in the nucleus.

Aromatase, estrogen receptors and brain development in fish and amphibians

Estrogens affect brain development of vertebrates, not only by impacting activity and morphology of existing circuits, but also by modulating embryonic and adult neurogenesis. The issue is complex as estrogens can not only originate from peripheral tissues, but also be locally produced within the brain itself due to local aromatization of androgens. In this respect, teleost fishes are quite unique because aromatase is expressed exclusively in radial glial cells, which represent pluripotent cells in the brain of all vertebrates. Expression of aromatase in the brain of fish is also strongly stimulated by estrogens and some androgens. This creates a very intriguing positive auto-regulatory loop leading to dramatic aromatase expression in sexually mature fish with elevated levels of circulating steroids. Looking at the effects of estrogens or anti-estrogens in the brain of adult zebrafish showed that estrogens inhibit rather than stimulate cell proliferation and newborn cell migration. The functional meaning of these observations is still unclear, but these data suggest that the brain of fish is experiencing constant remodeling under the influence of circulating steroids and brain-derived neurosteroids, possibly permitting a diversification of sexual strategies, notably hermaphroditism. Recent data in frogs indicate that aromatase expression is limited to neurons and do not concern radial glial cells. Thus, until now, there is no other example of vertebrates in which radial progenitors express aromatase. This raises the question of when and why these new features were gained and what are their adaptive benefits. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

The brain of teleost fish, a source, and a target of sexual steroids

Neurosteroids are defined as steroids de novo synthesized in the central nervous system. While the production of neurosteroids is well documented in mammals and amphibians, there is less information about teleosts, the largest group of fish. Teleosts have long been known for their high brain aromatase and 5α-reductase activities, but recent data now document the capacity of the fish brain to produce a large variety of sex steroids. This article aims at reviewing the available information regarding expression and/or activity of the main steroidogenic enzymes in the brain of fish. In addition, the distribution of estrogen, androgen, and progesterone nuclear receptors is documented in relation with the potential sites of production of neurosteroids. Interestingly, radial glial cells acting as neuronal progenitors, appear to be a potential source of neurosteroids, but also a target for centrally and/or peripherally produced steroids.

Immunohistochemical detection of estrogen receptors in fish scales

Calcium mobilization from internal stores, such as scales, induced by 17beta-estradiol during sexual maturation in salmonids is well documented. This calcium mobilization from scales is proposed to be mediated by the estrogen receptor (ER). However, the ER subtypes involved and signaling mechanisms responsible for this effect remain to be fully characterized. In the present study, we have localized ERalpha, ERbetaa and ERbetab proteins in juvenile and adult sea bream (Sparus auratus) and Mozambique tilapia (Oreochromis mossambicus) scales by immunohistochemistry with sea bream ER subtype specific antibodies. The three ERs were detected in isolated or small groups of round cells, in the basal layer of the scales of both juvenile and adult fish and the localization and signal intensity varied with the species and age of the animals. The ERs may be co-localized in cells of the scale posterior region that expressed tartrate-resistant acid phosphatase (TRAP), a marker for osteoclasts. These results suggest that the calcium mobilizing action of 17beta-estradiol on fish scales is via its direct action on ERs localized in osteoclasts.

Combination of receptor-binding assays and designed mutant receptors for discerning agonists and antagonists

Competitive receptor-binding assays are convenient for analyzing interactions between receptors and their ligands and for screening pharmaceutical drugs and potential endocrine-disrupting chemicals. Although these assays can be used for high-throughput screening, they cannot discern antagonists and agonists. Based on three-dimensional structures of complexes between ligand-binding domain of human estrogen receptor-alpha and its ligands, we designed mutant receptors with modified mode of ligand-binding. In the current studies we examined the binding of endogenous ligands, artificial ligands, and potent endocrine-disrupting chemicals to wild-type and Asp351 mutants of the human estrogen receptor-alpha ligand-binding domain. The new combination assay showed the decrease of relative biding affinity (RBA) values for antagonists. For example, RBA for tamoxifen was changed from 4.8 (using the Asp351 receptor) to less than 1.5 (using the Glu351 receptor). On the other hand, the agonists showed increase of RBA values. For example, RBA for bisphenol A was changed from 0.011 (using the Asp351 receptor) to less than 0.030 (using the Glu351 receptor). The variation of RBA was dependant on the type of mutant receptors. The change of RBA from wild-type to mutant-type can be an index for discerning agonists and antagonists. Comparison of RBA values obtained by assays using wild-type and mutant receptors is a simple way of discerning agonists and antagonists, and this approach could be extended to other types of receptors, if information of the receptors was enough to construct a designed mutant receptor.

Transcription of the fish Latent TGFβ-binding protein gene is controlled by estrogen receptor α

In endocrine disruption a key role has been suggested for endocrine receptors, in particular the estrogen receptors (ERs), in the regulation by compounds mimicking natural hormones. The two ERs, ERalpha and ERbeta are transcription factors involved in the regulated expression of estrogen target genes and have been shown to play an essential role in mammalian ovary development. A similar role is to be expected for ERs in fish; little is, however, known in fish about genes regulated by ERs. To begin to address this, we here report the identification and characterization of a novel gene regulated by the fish ERalpha in response to 17beta-estradiol. This gene encodes a fish orthologue of the latent transforming growth factor beta binding protein 3 (LTBP-3) and was identified through a differential display approach from a rainbow trout gonad cell line (RTG-2-ERalpha). We show that the rainbow trout LTBP (rtLTBP-3) is ERalpha dependent and is upregulated 5-fold in response to 17beta-estradiol addition. The rtLTBP shows 61% amino acid similarity to human LTBP-3 and 48%, 44% and 41% to LTBP-1, LTBP-2 and LTBP-4, respectively. The highly conserved TB2 domain of rtLTBP shows 87% and 66% identity to the TB domains of human LTBP-3 and LTBP-1, respectively. LTBP plays a pivotal role in TGFbeta activation in mammals and the high degree of sequence similarity suggests a similar role in fish. This would represent a novel link between nuclear hormone receptors and growth factor (TGFbeta) mediated developmental processes, and show new aspects of the role of hormones in developmental biology and endocrine disruption.

Intracellular Localization of hsp90 Is Influenced by Developmental Stage and Environmental Estrogens in Rainbow TroutOncorhynchus mykiss

In this study, we investigated the intracellular localization of heat shock proteins hsp90 and hsp70 in adult and juvenile rainbow trout (Oncorhynchus mykiss) and in juvenile trout exposed to estrogen or one of its mimics, 4-nonylphenol (4-NP). Livers were harvested from each group and analyzed directly or separated into nuclear and nonnuclear fractions. We found that hsp70 was predominantly nonnuclear in mature and juvenile fish regardless of treatment. Mature fish had significantly greater levels of hsp90 outside the nucleus, while juvenile fish had similar levels of hsp90 inside and outside the nucleus. Treatment with estradiol or 4-NP resulted in a translocation of hsp90 out of the nucleus in juvenile fish. To our knowledge, this is the first study to demonstrate a development- and/or estrogen-dependent shift in intracellular localization of hsp90 in fish. This change in subcellular distribution points to important roles for this hsp in fish estrogen signaling and development.

A biomarker approach to endocrine disruption in flounder--estrogen receptors, hepatocyte proliferation, and sperm motility

Three different physiological parameters were assessed to determine their potential for serving as biomarkers to predict abnormally elevated vitellogenin (VTG) production in male and immature flounder. Whereas abnormally elevated mean VTG plasma concentrations clearly distinguished the Mersey and Dee flounder studied, the results showed no significant differences in estrogen receptor binding capacity or binding affinity between the two groups. Hepatocyte proliferation was not found to be a "biomarker of effect" that could specifically be used to assess an increase in VTG-related proliferation. Nevertheless, immunohistochemical staining for proliferating cell nuclear antigen did show a significantly higher proliferative activity in the livers of Mersey flounder than that in Dee flounder. Sperm motility also was not found to be a biomarker of effect linked to an abnormal elevation of VTG plasma concentration. The results (higher sperm quality in terms of motility in Mersey flounder) were unexpected but interesting.

Effects of cadmium on the reproductive axis of Japanese medaka (Oryzias latipes)

Cadmium (Cd) is a ubquitous element and a significant inorganic pollutant that has previously been found to bioaccumulate in reproductive organs of fish and disrupt important endocrine processes, especially those involved in synthesis, release and metabolism of hormones. Clearly, there is potential for reproductive effects in fish populations exposed to Cd, however, few studies have investigated the non-lethal consequences of Cd in fish. To this extent, adult male and female Japanese medaka were exposed to 0-10 ppb Cd for 7 weeks. Reproductive endpoints were monitored during weeks 6 and 7 of exposure and compared to physiological responses along the hypothalamus-pituitary-gonadal (HPG) axis, including plasma vitellogenin (VTG), hepatic estrogen receptor (ER), plasma steroids, gonadal-somatic indices (GSI), and gonadal steroid release. There were no observed effects on VTG and ER by long-term Cd exposure. However, gonadal steroid release was significantly decreased in males and females at all exposure concentrations and female plasma estradiol levels were significantly altered at concentrations higher than 5 ppb Cd. Overall, responses along the HPG axis were more sensitive to Cd exposure than the reproductive and developmental endpoints, which were not affected in this study, indicating that higher level impairment in fish might be relatively protected.

Molecular cloning, characterisation, and tissue distribution of oestrogen receptor alpha in eelpout (Zoarces viviparus)

A cDNA encoding the eelpout (Zoarces viviparus) oestrogen receptor alpha (eERalpha) has been isolated from eelpout liver, cloned and sequenced. The cDNA contains a complete open reading frame encoding 570 amino acid residues (mw: 63.0 kDa). The amino acid sequence of eERalpha showed a high degree of identity to ERalpha of other teleost species. The tissue distribution of eERalpha mRNA was examined using Northern blotting, RT-PCR and in situ hybridisation (ISH). All three methods identified a pronounced expression of eERalpha in liver, pituitary, testis and ovary. In the brain ISH experiments showed that ERalpha mRNA was highly expressed in distinct regions of the preoptic area and the mediobasal hypothalamus. We have provided evidence that the receptor is auto-regulated by 17beta-oestradiol (E(2)) not only in liver but also in the testis, indicating an important role for E(2) during spermatogenesis in male eelpout. RT-PCR analysis showed a broader expression pattern including significant expression in the brain, kidney, heart, and gut of adult eelpout. In eelpout embryos eERalpha expression has also been identified, indicating a possible role for the receptor in early development. This study contributes to the accumulating evidence that in fish E(2) is not only involved in the regulation of liver specific proteins, but has a much broader range of targets.

Influence of parental and developmental cadmium exposure on endocrine and reproductive function in Japanese medaka (Oryzias latipes)

Cadmium (Cd) is a ubiquitous element and an important anthropogenic metal contaminant. A series of assays were modified or developed for Japanese medaka (Oryzias latipes), and used to compare the effects of Cd exposure on indicators of endocrine function in adult animals previously exposed in ovo or as hatchlings. Adults were raised either from eggs produced during a 2 week exposure to 0-10 microg/l Cd or from fry exposed for 2 weeks beginning 2 days after hatching. The reproductive capacity of the resulting adults was determined during a 2 week period during which half of the animals were re-exposed to Cd. Two week Cd exposure did not result in reproductive impairment despite producing some changes in circulating steroid concentration. In addition, 1 microg/l cadmium exposure in ovo elevated male hepatic vitellogenin (VTG) relative to controls. Hence, steroid parameters were a better biomarker of cadmium exposure than changes in VTG. However, reproductive impairment was not correlated to change in VTG or plasma steroids.

Localization of tyrosine hydroxylase and its messenger RNA in the brain of rainbow trout by immunocytochemistry and in situ hybridization

This report describes the distribution of tyrosine hydroxylase (TH)-expressing structures in the brain of rainbow trout (Oncorhynchus mykiss). TH neurons have been localized by the use of two complementary techniques, immunocytochemistry and in situ hybridization of TH messenger RNA. Results obtained from in situ hybridization and immunocytochemistry were in agreement. TH cells were observed in many areas of the brain, with a higher density at the level of the olfactory bulbs where TH-positive neurons are abundant in the internal cell layer. In the telencephalon, two populations of TH neurons can be distinguished: one group is located in the area ventralis telencephali pars dorsalis, and the other group is located in the area ventralis telencephali pars ventralis and extends laterally in the area ventralis telencephali pars lateralis. Many labeled neurons are also seen in the preoptic area as well as in the hypothalamus, where several clusters of TH-positive cells are observed. Some of these neurons located in the paraventricular organ grow a short cytoplasmic extension directed to the ventricular wall and are known to be cerebrospinal fluid-contacting cells. The most caudal TH neurons are observed at the level of the locus caeruleus. At the level of the pituitary, TH-positive fibers are observed in the neurohypophysis. The TH-immunoreactive innervation at the level of the pituitary provides a neuroanatomic basis for the effects of dopamine and/or norepinephrine on the release of pituitary hormones in fish.

Double evaluations of chemicals using a cocktail of fused recombinant receptors

Some chemicals have multipotential as endocrine-disrupting chemicals (EDCs). For example, some chemicals act as both estrogens and antiandrogens. Numbers of such chemicals should be evaluated from many aspects; however, labor and expenses are generally limited. We have developed two expression systems for the wild type of human estrogen receptor alpha and the wild type of human androgen receptor fused with a maltose binding protein. They are soluble and have binding activities. They showed dose-responses to natural hormones and well-known potential EDCs. After we established each assay condition for a competitive binding assay using each receptor, we found that two assay systems can be carried out simultaneously under limited and harmonized conditions. Under harmonized conditions using a cocktail of two types of receptors, we could estimate natural hormones and potential EDCs. Interference between two assay systems was not observed under these conditions. We believe that some competitive binding assays can be carried out using a cocktail of receptors at the same time if interference among different assay systems can be avoided by choosing ideal conditions.

Tissue-specific expression of two structurally different estrogen receptor alpha isoforms along the female reproductive axis of an oviparous species, the rainbow trout

In oviparous species, in addition to a full-length estrogen receptor alpha (ER alpha), another ER alpha isoform lacking the A domain and exhibiting a ligand-independent transactivation function has been consistently reported. Although both isoforms are expressed in the liver, their respective sites of expression in other potential target tissues are unknown. In contrast to the situation in Xenopus and chicken, the two isoforms of rainbow trout (Oncorhynchus mykiss) are generated from two classes of transcripts with different 5' untranslated sequences issued from the same gene by alternative splicing and promoter usage. The aim of this study was to take advantage of the unique organization of the rainbow trout ER alpha gene to investigate the tissue distribution of these two messenger species along the reproductive axis of female trout. The S1 nuclease assay and in situ hybridization were used, with probes specific for each of the transcripts. Reverse transcription polymerase chain reaction (RT-PCR) with primers specific for each of the isoforms also was performed. The data indicated that the full-length ER alpha is expressed in liver, brain, pituitary, and ovary, whereas expression of the isoform with the truncated A domain is restricted to the liver, demonstrating a tissue-specific expression of these two ER alpha isoforms. The presence of a short liver-specific isoform in oviparous species suggests its role in the development and/or maintenance of the unique function of the liver in the vitellogenesis process.

Estrogen receptor-alpha protein localization in the testis of the rainbow trout (Oncorhynchus mykiss) during different stages of the reproductive cycle

Estrogen receptor-alpha (ER-alpha) is important for male reproduction in mammals; however, no information is available on ER-alpha protein distribution in the testes of fishes. The cellular localization of the rainbow trout (Oncorhynchus mykiss) ER-alpha (rtER-alpha) protein, throughout the annual reproductive cycle was determined in this study. An antibody was designed based on a 15-amino acid sequence from the D-domain of the rtER-alpha, and its specificity was confirmed using Western blot analysis. Immunohistochemical analysis revealed rtER-alpha protein to be present only in the testicular interstitium, at every stage of the annual reproductive cycle. The localization of rtER-alpha protein in the interstitial fibroblasts, the Leydig cell precursor in the rainbow trout, suggests a role for estrogens in the differentiation of these precursor cells into mature Leydig cells. This is the first study to report the cellular localization of an estrogen receptor protein in the testis of any fish species.

The presence of high-affinity, low-capacity estradiol-17beta binding in rainbow trout scale indicates a possible endocrine route for the regulation of scale resorption

High-affinity, low-capacity estradiol-17beta (E(2)) binding is present in rainbow trout scale. The K(d) and B(max) of the scale E(2) binding are similar to those of the liver E(2) receptor (K(d) is 1.6 +/- 0.1 and 1.4 +/- 0.1 nM, and B(max) is 9.1 +/- 1.2 and 23. 1 +/- 2.2 fmol x mg protein(-1), for scale and liver, respectively), but different from those of the high-affinity, low-capacity E(2) binding in plasma (K(d) is 4.0 +/- 0.4 nM and B(max) is 625.4 +/- 63. 1 fmol x mg protein(-1)). The E(2) binding in scale was displaced by testosterone, but not by diethylstilbestrol. Hence, the ligand binding specificity is different from that of the previously characterized liver E(2) receptor, where E(2) is displaced by diethylstilbestrol, but not by testosterone. The putative scale E(2) receptor thus appears to bind both E(2) and testosterone, and it is proposed that the increased scale resorption observed during sexual maturation in both sexes of several salmonid species may be mediated by this receptor. No high-affinity, low-capacity E(2) binding could be detected in rainbow trout gill or skin.

Two estrogen receptor (ER) isoforms with different estrogen dependencies are generated from the trout ER gene

A characteristic of all estrogen receptors (ER) cloned from fish to date is the lack of the first 37-42 N-terminal amino acids specific to the A domain. Here we report the isolation and characterization from trout ovary of a full-length complementary DNA (cDNA) clone encoding an N-terminal variant form of the rainbow trout ER (rtER). Sequence analysis of open reading frame of this cDNA predicts a 622-amino acid protein. The C-terminal region of this protein, from amino acid position 45 to the end, was very similar to the previously reported rtER (referred to as the short form, or rtER(S)). In contrast, this novel rtER cDNA (referred to as the long form, or rtER(L)) contains an additional in-frame ATG initiator codon that adds 45 residues to the N-terminal region of the protein. This new N-terminal region may represent the A domain of ER found in tetrapod species. The first 227 bp of this new cDNA were similar to the 3'-end intronic sequence of the rtER gene intron 1. These data together with S1 nuclease, primer extension, and RT-PCR experiments demonstrate that the rtER(L) represents a second isoform of rtER that arises from an alternative promoter within the first intron of the gene. Transcripts encoding both rtER forms were expressed in the liver. In vitro translation of the rtER(L) cDNA produced 2 proteins with molecular masses of 71 and 65 kDa, whereas rtER(S) cDNA produced 1 65-kDa protein. Interestingly, Western blot analysis with a specific antibody against the C-terminal region of rtER revealed 2 receptor forms of 65 and 71 kDa in trout liver nuclear extracts, in agreement with the presence of the 2 distinct classes of rtER messenger RNA in this tissue. Functional analysis of both rtER isoforms revealed that although rtER(S) consistently exhibited a basal (estrogen-independent) trans-activation activity that could be further increased in the presence of estrogens, the novel isoform rtER(L) is characterized by a strict estrogen-dependent transcriptional activity. These data suggest that the additional 45 residues at the N-terminal region of rtER(L) clearly modify the hormone-independent trans-activation function of the receptor.

Cloning of fish enzymes and other fish protein genes

Fish metabolism needs special enzymes that have maximum activity at very different conditions than their mammalian counterparts. Due to the differences in activity, these enzymes, especially cold-adapted proteases, could be used advantageously for the production of some foods. In addition to the enzymes, this review describes some other unique fish polypeptides such as antifreeze proteins, fluorescent proteins, antitumor peptides, antibiotics, and hormones, that have already been cloned and used in food processing, genetic engineering, medicine, and aquaculture. Recombinant DNA technology, which allows these biological molecules to be cloned and overexpressed in microorganisms is also described, highlighting innovative applications. The expected impact of cloning fish proteins in different fields of technology is discussed.

Two functionally different protein isoforms are produced from the chicken estrogen receptor-alpha gene

The existence of two forms of the chicken estrogen receptor-alpha protein (ER-alpha) in chicken tissues is demonstrated: the previously reported receptor (cER-alpha form I), which has a size of 66 kDa, and a new form (cER-alpha form II), which lacks the N-terminal 41 amino acids present in form I and thus gives rise to a protein of 61 kDa. Whereas the 66-kDa protein is the translation product of several cER-alpha mRNAs (A1-D), the cER-alpha protein isoform II is encoded by a new cER-alpha mRNA (A2), which is transcribed in vivo from a specific promoter that is located in the region of the previously assigned translation start site of the cER-alpha gene. SI nuclease mapping analysis reveals that cER-alpha mRNA A2 is liver enriched. The resulting cER-alpha forms I and II differ in their ability to modulate estrogen target gene expression in a promoter- and cell type-specific manner. Whereas cER-alpha form I activates or represses in a strictly E2-dependent manner, the truncated form is characterized by a partial transactivating or repressing activity in the absence of its ligand. Comparison of the N-terminal coding regions of different vertebrate ER-alpha reveal a conservation of the translation start methionine of the protein ER-alpha form II in other oviparous species but not in mammals. The expression of two classes of ER-alpha transcripts encoding the two ER-alpha receptor forms in the liver of Xenopus laevis and rainbow trout is demonstrated. Therefore, the existence of two functionally different protein isoforms produced from the ER-alpha gene is probably a common and specific feature in oviparous species.

Synergism between a half-site and an imperfect estrogen-responsive element, and cooperation with COUP-TFI are required for estrogen receptor (ER) to achieve a maximal estrogen-stimulation of rainbow trout ER gene

In all oviparous, liver represents one of the main E2-target tissues where estrogen receptor (ER) constitutes the key mediator of estrogen action. The rainbow trout estrogen receptor (rtER) gene expression is markedly up-regulated by estrogens and the sequences responsible for this autoregulation have been located in a 0.2 kb upstream transcription start site within - 40/- 248 enhancer region. Absence of interference with steroid hormone receptors and tissue-specific factors and a conserved basal transcriptional machinery between yeast and higher eukaryotes, make yeast a simple assay system that will enable determination of important cis-acting regulatory sequences within rtER gene promoter and identification of transcription factors implicated in the regulation of this gene. Deletion analysis allowed to show a synergistic effect between an imperfect estrogen-responsive element (ERE) and a consensus half-ERE to achieve a high hormone-dependent transcriptional activation of the rtER gene promoter in the presence of stably expressed rtER. As in mammalian cells, here we observed a positive regulation of the rtER gene promoter by the chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI) through enhancing autoregulation. Using a point mutation COUP-TFI mutant unable to bind DNA demonstrates that enhancement of rtER gene autoregulation requires the interaction of COUP-TFI to the DNA. Moreover, this enhancement of transcriptional activation by COUP-TFI requires specifically the AF-1 transactivation function of ER and can be observed in the presence of E2 or 4-hydroxytamoxifen but not ICI 164384. Thus, this paper describes the reconstitution of a hormone-responsive transcription unit in yeast in which the regulation of rtER gene promoter could be enhanced by the participation of cis-elements and/or trans-acting factors, such as ER itself or COUP-TF.

Identification of potential sites of cortisol actions on the reproductive axis in rainbow trout

The full length cDNA encoding a rainbow trout glucocorticoid receptor (rtGR) has been obtained from rainbow trout liver and intestine libraries. Northern blot analysis showed that the corresponding messengers are detected in the brain of trout with a size 7.5 kb similar to the size of rtGR mRNA in other target tissues. The distribution of the rtGR mRNA and protein was studied in the forebrain of the trout by means of both in situ hybridization and immunohistochemistry and compared with that of the oestrogen receptor (rtER). The GR and ER mRNAs and proteins were detected with a strong overlapping mainly in the: (a) preoptic region; (b) mediobasal hypothalamus; and (c) anterior pituitary, confirming their implication in the neuroendocrine control of pituitary functions. In both diencephalon and pituitary, the peptidergic phenotype of some neuron or cell categories expressing either type of receptors could be determined by double staining. Furthermore, double staining studies have demonstrated colocalization of the two receptors in the same neurons or pituitary cells. The rtER and rtGR were found to be co-expressed in the dopaminergic neurons inhibiting GTH2 secretion and in pituitary cells of the anterior lobe--notably the gonadotrophs. Given that the promoter of the ER gene contains several potential glucocorticoid-responsive elements (GRE) and that cortisol inhibits the oestradiol-stimulated ER expression in the liver, the possibility exists for modulation of ER gene expression by GR in the hypothalamo-pituitary complex. This could explain some of the well documented effects of stress on the reproductive performance in salmonids.

Rainbow trout glucocorticoid receptor overexpression in Escherichia coli: production of antibodies for western blotting and immunohistochemistry

Fragments of cDNA that encode the N-terminal and DNA-binding domains (DBD) of the rainbow trout glucocorticoid receptor (rtGR) were expressed in Escherichia coli as fusion proteins with glutathione-S-transferase (GST). The fusion proteins induced by IPTG could readily be detected as 45- and 40-kDa bands, respectively, in crude extracts, as well as in proteins purified on glutathione-agarose. These purified hybrid proteins were used to immunize rabbits. The antisera produced were tested for specificity by Western blot analysis using extracts from COS-1 cells transfected with an rtGR expression vector and from trout liver cells. The antisera raised against the DBD domain did not detect any bands on Western blots, even at low antiserum dilution. However, the purified DBD fusion protein specifically bound GRE-containing DNA fragments in gel-shift assays, and the retarded complexes were supershifted by these antibodies. The antisera raised against the N-terminal domain consistently detected two protein bands at 104 and 100 kDa in the two cell extracts and allowed specific immunohistochemical staining in fish brain and pituitary. For the first time in fish, these antibodies will allow analysis of GR expression in different cortisol target tissues.

Differential regulation of two genes implicated in fish reproduction: vitellogenin and estrogen receptor genes

In rainbow trout as well as in other species, variability of estrogen receptor (ER) gene expression according to the cell type and the physiological state reflects a differential cell and gene sensitivity to estrogen. We previously demonstrated that expression of the rainbow trout estrogen receptor (rtER) and vitellogenin (Vg) genes were induced differently by estrogens in rainbow trout liver. Therefore, these two genes offered a suitable model to study the influence of ER concentration on gene transcriptional activities. In the present study we show that the transcription rate of rtER and Vg genes during an estrogenic treatment are affected differently by variation of cellular ER concentration. We demonstrate that rtER gene exhibits a low threshold response to loaded estrogen receptor, and increasing ER amounts do not affect the transcriptional response of this gene during an estrogenic stimulation. On the contrary, Vg gene expression requires the presence of a higher loaded estrogen receptor level to be induced, and its transcriptional response is directly proportional to the amount of synthesised ER.

Estrogen receptor mRNA in mineralized tissues of rainbow trout: calcium mobilization by estrogen

RT-PCR was undertaken on total RNA extracts from bone and scales of the rainbow trout, Oncorhynchus mykiss. The rainbow trout estrogen receptor (ER)-specific primers used amplified a single product of expected size from each tissue which, using Southern blotting, strongly hybridized with a 32P-labelled rtER probe under stringent conditions. These data provide the first in vivo evidence of ER mRNA in bone and scale tissues of rainbow trout and suggest that the effects of estrogen observed in this study (increased bone mineral and decreased scale mineral contents, respectively) may be mediated directly through ER.

Up-regulation of estrogen receptor mRNA and estrogen receptor activity by estradiol in liver of rainbow trout and other teleostean fish

Injection of estradiol (E2) into immature rainbow trout resulted in the induction of the hepatic vitellogenin gene mediated by the nuclear estrogen receptor (ER). Liver ER mRNA rose markedly on E2 treatment in three groups of trout kept at different temperatures. Only in the group kept at 4 degrees C did the total cellular ER, as measured by [3H]estradiol-binding activity in nuclear and cytosol fractions, parallel the ER mRNA level. In fish kept at 9 degrees C and 15 degrees C, the ratio of total ER activity to ER mRNA fell during chronic E2 treatment, probably reflecting translational of post-translational control mechanisms. Upregulation of ER mRNA also occurred in sea raven, sculpin, winter flounder, and Atlantic salmon after E2 treatment. Intrahepatic ER activity rose proportionately in Atlantic salmon kept at 6-9 degrees C but not in sea raven, sculpin, or flounder. We conclude that the regulation of ER expression in teleosts is complex and includes transcriptional, translational, and post-translational elements and is influenced by environmental temperature.

Do gonadotrophin-releasing hormone neurons express estrogen receptors in the rainbow trout? A double immunohistochemical study

A double immunocytochemical procedure, with two different chromogens, was used to compare the respective distributions of estrogen receptor-immunoreactive cells and gonadotrophin-releasing hormone-immunoreactive neurons on the same sections of the brains of adult male and female rainbow trout (Oncorhynchus mykiss). Estrogen receptor-immunoreactive cells were observed in the ventral and lateral telencephalon, the preoptic region, the mediobasal hypothalamus, and the ventromedial thalamic nucleus. Gonadotrophin-releasing hormone-immunoreactive perikarya were detected in the olfactory bulbs, the ventral telencephalon, the preoptic area, and the mediobasal hypothalamus. Double-staining studies showed that, although some estrogen receptor-positive cells were in close proximity to gonadotrophin-releasing hormone-immunoreactive perikarya, careful examination of 550 gonadotrophin-releasing hormone-positive cells from five adult females and two adult males failed to demonstrate any evidence that gonadotrophin-releasing hormone neurons coexpress estrogen receptor in the brain of the rainbow trout. The present study provides, for the first time in teleosts, morphological evidence that gonadotrophin-releasing hormone neurons do not represent major direct targets for estradiol, suggesting that the positive feedback effects of estradiol onto the gonadotrophin-releasing hormone system are likely to be conveyed via other cell populations.

Differential functional activities of rainbow trout and human estrogen receptors expressed in the yeast Saccharomyces cerevisiae

The cDNA of rainbow trout estrogen receptor (rtER), highly and stably expressed in yeast, Saccharomyces cerevisiae, was used to analyse the biological activity of the receptor. The rtER mRNA encoded a 65-kDa protein which was immunorevealed by a specific antibody and migrated with the authentic rtER major protein form detected in trout liver. Yeast rtER bound estradiol with high affinity and the dissociation constant (Kd = 1.35 nM) was very similar to the value measured from trout liver extracts but 3-5-fold higher than the Kd found for human estrogen receptor (hER). This indicates therefore that the rtER has a lower estradiol affinity compared to the human receptor. While the hER Kd remained unchanged at both 4 degrees C or 22 degrees C, it was slightly modified at 30 degrees C. The Kd measured for rtER at 22 degrees C and 30 degrees C were about 2-fold, and 12-fold higher, respectively, than the Kd obtained at 4 degrees C suggesting an alteration of the rtER affinity for its ligand at elevated temperature. To examine the estrogen-receptor-mediated activation of transcription in yeast, reporter plasmids integrated or not in the yeast genome were used. The reporter genes consist of one, two, or three copies of estrogen-responsive elements (ERE) upstream of the yeast proximal CYC1 or URA3 promoters fused to the lacZ gene of Escherichia coli coding for beta-galactosidase. The induction of beta-galactosidase activity for all reporter genes was strictly dependent on the presence of rtER and estrogens. The activation of transcription mediated by rtER responded in an estradiol-dose-dependent manner as in animal cells. However, compared to hER, the estradiol concentration necessary to achieve maximal activation was 10-fold higher. This is probably a consequence of the lower estradiol-affinity for rtER compared to hER. The levels of induction of the reporter genes containing two or three ERE were strongly enhanced compared to the one ERE construct. This is in agreement with the synergistic effect previously described for multiple ERE. The magnitudes of transcriptional induction mediated by rtER and hER were similar when the reporter gene containing three ERE was used but changed when the one ERE construct was used. In this case transcriptional activation indicated by rtER was 10-20 fold lower. This suggests that rtER requires protein/protein interaction for its stabilization on DNA. Antiestrogens were able to bind rtER and promote gene transcription. However, to produce effects comparable to those obtained with estrogens, much higher concentrations were required. This may imply nonetheless that antihormones were capable of provoking efficient interactions of rtER with the transcriptional machinery.

Rainbow trout estrogen receptor presents an equal specificity but a differential sensitivity for estrogens than human estrogen receptor

The estrogen receptor is a transcription factor that mediates the actions of estrogens which plays a major role in salmonid vitellogenesis. Previously, we cloned and described the estradiol receptor in rainbow trout. To study the transactivation characteristics, the full length cDNA was inserted in an expression vector and tested by transfection on an estrogen-dependent promoter (pERE-TK-CAT). For the first time, direct comparison between the trout receptor (rtER) and human receptor (hER) in an analogous system has been possible. We demonstrate that rtER can, like hER, transactivate transcription in a strictly hormone-dependent manner. The specificity and sensitivity of the rtER response to different steroids have been studied. With rtER, the E2 concentration needed for half maximal activation is 10 times higher than with hER. In addition, we show that rtER has a weak affinity for androgens and transactivation could be induced using high testosterone concentration. Experiments show that both rtER and hER have an equal specificity for estrogens but that the rtER binds its ligand with a lower sensitivity than hER. Several hypotheses concerning the role of main amino acids within the receptor protein are proposed based on the different properties of the receptors and comparison of sequences.

Distribution of estrogen receptor-immunoreactive cells in the brain of the rainbow trout (Oncorhynchus mykiss)

Using antibodies against the hormone binding domain of the trout estrogen receptor (ER), the distribution of ER-immunoreactive (ER-IR) cells was studied in the brain of maturing diploid and triploid female rainbow trout using a streptavidin-biotin-peroxidase method followed by a nickel-intensified diaminobenzidine reaction. This technique resulted in an excellent signal/background ratio allowing unambiguous identification of positive cells. In all animals, ER-IR cells were consistently located in three brain regions, the ventral telencephalon, the anterior ventral preoptic region, and the mediobasal hypothalamus. About 250 ER-IR cells were observed in the ventral and dorsal parts of the ventral telencephalon. In the anterior nucleus preopticus periventricularis, about 2400 ER-IR cells were observed surrounding the preoptic recess. In the posterior hypothalamus, approximately 2700 ER-IR cells were located in the anterior, posterior and inferior divisions of the nucleus lateralis tuberis and in the nucleus saccus vasculosus. In these regions cell nuclei exhibiting different densities of staining were observed and absolutely no labeling of cytoplasmic processes was detected. These results are in partial agreement with those obtained either after injection of tritiated-estradiol in other teleots species or in situ hybridization of ER mRNAs in trout. In particular, no immunoreactivity was observed in the thalamic region nor in the nucleus posterioris periventricularis. These data indicate that target cells for estradiol are essentially located in brain regions involved in the neuroendocrine control of pituitary functions and having direct connections with the hypophysis.